Certain industrial manufacturing processes are carried out in the presence of a volatile solvent which vaporizes as it is used. Merely by way of example, such volatile solvents are used during the manufacture of pharmaceuticals. Typically, such manufacturing processes are "ventilated" using an inert gas such as nitrogen to carry away the vaporized solvent. In some instances, it had been the practice to exhaust the gas with its entrained, vaporized solvent through a conduit such as an exhaust stack and release it to the ambient atmosphere.
More recently, environmental studies indicate (or at least strongly suggest) that vaporized solvents which are so released have an adverse effect upon the upper atmosphere. As a consequence, refrigeration systems, including those of the cascade type, have been developed to recover vaporized solvent.
One such system, which may be thought of as a separation system, uses a precooler such as a water-ethylene glycol chiller to reduce the temperature of the inert gas-vaporized solvent mixture flowing in the stack. Such temperature is reduced from a nominal ambient temperature of about 70.degree.-80.degree. F. to a level slightly above the freezing point of water, i.e., to about 35.degree. F., to remove moisture from such mixture. For further temperature reduction, a plurality of evaporator heat transfer units is arranged downstream of the precooler.
Such transfer units are connected to the refrigeration system and disposed in a heat transfer relationship to the mixture. These transfer units result in the direct transfer of heat from the mixture of vaporized solvent and gas to the units. As used herein, "direct transfer" and like phrases means transfer of heat between a medium (such as the gaseous mixture) and a refrigerant without using other heat-carrying media. As a result of such heat transfer, the temperature of the mixture (including the vaporized solvent) decreases to levels well below 0.degree. F. and such solvent is thereby condensed to a liquid. The solvent is thus separated from the gaseous mixture. Once the solvent is "stripped" from the mixture, the inert gas (now substantially free of vaporized solvent) is vented to atmosphere through the exhaust end of the stack.
Another, quite different kind of separation system is shown in U.S. Pat. No. 1,805,400 (Hirsch). Such system has two compressors in parallel or in series, depending upon the embodiment, and uses the same type of refrigerant in both compressors. The system is used to separates product such as wine, oil, paraffin, peat or cider into constituents by chilling such products. The system has a low pressure main condenser cooled by a separated cooled or frozen constituent, ice being an example of the latter. It appears that such constituent provides inadequate cooling, even when the constituent is ice, melting of which is apparently to be avoided. Therefore, additional cooling is required in the form of a high pressure auxiliary condenser which is generally in parallel with the low pressure condenser. Such high pressure condenser is cooled by water. Supplemental cooling fans are also suggested. In the only example described in any detail, the entire product, water-bearing peat sods, rather than a constituent thereof is used for cooling. That is, cooling is part of the process used to separate water from the sods rather than a result of using a post-separation constituent for cooling.
The described operation of the Hirsch system suggests that if cooling does not occur at the low pressure main condenser, the system becomes "self-impairing" if not actually inoperative for its essential purpose at least as to the separation of water from peat sods. Further, system temperatures are insufficiently low for recovering vaporized solvent. And the system is likely to be ungainly if not unworkable for solvent separation and recovery at least to the extent that ice or peat sods are used as the cooling medium.
Such earlier systems fail to recognize and utilize the full cooling potential of the recovered constituent. In particular, such systems do not evidence an appreciation of how a recovered constituent such as a very cold solvent may be used to increase refrigerant efficiency in a cascade-type solvent recovery system. And such earlier systems, the Hirsch system for example, may become functionally impaired or inoperative under certain circumstances if cooling at the low pressure main condenser does not occur.